Process and salt for descaling of metals



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3,936,239 PROCE AND SALT F-GR DESCALENG F METAL Matthew Mekjean, Niagara Falls, and Charles A. Strack,

Lewiston, N.Y., assignors to Hooker Chemical Corporation, a corporation of New York No Drawing. Filed Oct. 24, 1957, Ser. No. 692,044 6 Claims. (Ql. 134-49) This invention relates to the art of removing oxide scales from Work pieces of metal, and in particular removing the oxides of molybdenum from the metal and its alloys, by immersion in a molten descaling salt mixture at elevated"-temperatures.

There is a sparsity of prior art dealing with the descaling and cleaning of molybdenum by chemical means. Molybdenum cannot be cleaned and brightened in the conventional pickling acids. Even nitric-hydrofluoric acid,

in normal concentrations has little or no effect in removing the tough, black oxide that is created in the annealing furnace. Other molten salt baths available to the industry either destroy scale and metal, or have no effect whatever. Oxidizing salt baths, popular for the successful descaling of stainless steels and titanium, have been found unsatisfactory for descaling molybdenum as they destroy scale and metal in an almost uncontrollable exothermic reaction. Chemically reducing molten salt baths, used in industry for removing scale from ferrous steels, alloys and varieties of stainless steel has not proven acceptable for descaling molybdenum, because the work pieces are left dark brown to purple and also the cycles are quite long.

It is an object of this invention to provide a molten salt bath which descales' and brightens molybdenum on all surfaces. It is another object of this invention to provide a molten salt bath which leaves the descaled and brightened molybdenum completely smooth and clean with no sign of etching and essentially without metal loss. It is a further object of this invention to provide a molten salt bath which accomplishes these results in a short period of time and is simply and expeditiously handled. It is still a further object of this invention to descale molybdenum without necessitating the use of electricity, by a simple dip method, although electrolytic means can be used. Further objects and purposes of this invention will appear herein.

We have now found a substantially anhydrous salt bath for descaling, brightening and cleaning of metals comprising a mixture of (l) a major portion of alkali hydroxide and (2) a minor portion of (a) alkali phosphate, (b) alkali carbonate and (c) alkali halide. We have also found a process for removing the contaminated high oxygen-containing surface layer and annealed scale of metals and their alloys, and in particular molybdenum. By a simple dip method molybdenum can be completely desca.ed, brightened and cleaned in our molten salt without attack on the metal. These results are particularly unexpected in view of the fact that molten caustic alk alis dissolve molybdenum in the absence of oxygen, beginning at 660 C., and at lower temperatures in the presence of oxygen. The versatility of the salt bath will be indicated from its other uses. These will become apparent herein.

The alkali hydroxides which may be used include sodium, potassium, lithium, rubidium and the other alkali metals, as well as mixtures thereof. Because of availability and lower cost, however, sodium hydroxide is preferred in this composition, but this is not to be taken as limiting the scope of the invention in any Way. It is preferable to use caustic soda because it is commercially available at a relatively low cost.

In use the molten salt bath slowly absorbs carbon dioxide from the atmosphere and converts some of the alkali hydroxide to alkali carbonate. Another source of carbon dioxide in the proximity of the bath would be the exhausts of gas or oil-fired heating tubes for maintaining the salt in the molten state. This conversion of alkali hydroxide to alkali carbonate in no way adversely affects the ability of the salt to descale molybdenum due to the great'excess of caustic that is present in the salt bath.

The caustic constitutes the major portion of the salt bath and the composition of the caustic in the salt bath may be varied from about 50 to 90 percent by weight of the total mixture. The equilibrium composition reached in a commercially operated bath will be determined in each case by the tonnage of metal which the bath descales in continuous operation. In other words, the dragout of salt which wets the surface of the Work being descaled, as well as the rack materials holding the work, would constitute a loss of salt from the bath and lost in the quench water. This loss of salt is periodically replaced by the addition of fresh material. This automatically maintains the equilibrium composition of the bath. Actual experience has indicated the preferred composition of caustic in the salt bath to be from about to percent by weight of the total mixture. 1

The alkali phosphates which may be used include sodium, potassium, lithium, rubidium, the other alkali metals, alkaline earths like calcium, magnesium and barium for example, and mixtures thereof. Because of availability and lower cost the phosphates of sodium are preferred. These would substantially include sodium dihydrogen phosphate NaH PO trisodium phosphate (Na PO sodium metaphosphate (NaPO disodium. hydrogen phosphate (NaH PO trisodium phosphate These reactions are intended merely as illustrative and should not be taken as limiting the scope of this invention. The presence of the phosphate in the molten bath tends to form a protective transparent film of metallic phosphate which functions to inhibit attack on the base metal, but does not retard the dissolution of the oxide scale on the work piece.

The concentration of the phosphatein the molten solution may be varied over a wide range. A maximum of 25 percent and a minimum of two percent alkali phosphate by Weight of the total mixture may be used. The preferred range of concentration however, is from four percent to twelve percent by Weight of the total mixture.

The alkali carbonates which may be used include sodium, potassium, lithium, rubidium, the other alkali metals, the alkaline earth carbonates, and mixtures thereof. Due to economic considerations, availability and solubility of these carbonates in the molten salt, the preferential carbonates in the molten solution are the sodium carbonate, potassium carbonate and mixtures thereof. As pointed out previously, the caustic alkali, which is substantially the major part of the-bath, will slowly react with the carbon dioxide in contact with the molten surface to form the carbonate. This in no way limits the effectiveness of the bath to descale the molybdenum due to the great excess of caustic that is present.

The concentration of the carbonates present may be varied over a wide range. The minimum is approximately one half of one percent to a maximum of approximately thirty percent by weight of the total mixture. Higher concentrations may be used but at uneconomical temperatures. The preferred range of concentration varies from about one half of one percent to about twenty percent by weight of the total mixture. The alkali halides which may be used include sodium, potassium, lithium, rubidium, the other alkali metals, alkaline earth halides such as calcium, barium and magnesium for example, and mixtures thereof. Sodium and potassium halides and preferentially mixtures thereof are the preferred alkali halides because they are economical. All halides may be advantageously used although chloride is preferred due to economic considerations. Fluoride which may be advantageously used is not preferred because of the economic considerations and also because it requires the use of a relatively expensive ventilating apparatus for the protection of op erating personnel because of volatile toxic fluorides created in the bath. The total range of composition of alkali halides may be varied from about one half of one percent to twenty-five percent by weight of the total mixture.

The special additives which may be included to achieve more desirable properties for specific applications include stannates, borates, sulfates, sulrites, hydrides, nitrates, nitrites, silicates, molybdates, chromates, dichromates, fiuosilicates, fluoborates, manganates, permanganates, reaction products thereof, decomposition products thereof, and mixtures thereof. It is preferred to use the alkali metal additives since this corresponds to the bath. However, others can be used. The use of these additives would be apparent to one versed in the art.

The temperatures at which the molten salt may be economically used can be varied over a range between about 650 F., to about 1250 F., although the preferred operating temperature would be in the range of from about 800 F. to about 1000 F. The bath may be kept in the molten state by the conventional methods known to those skilled in the art. For example small furnaces might be economically controlled by electric heating. Larger commercially operated baths, which would have high heat dragout, might be more economically heated by gas fired immersion tubes. Other heating methods which appear convenient may be uti lized.

In all cases the specific volume of the metal being treated and the thickness of the scale on the work piece willcontrol the time of the descaling cycle. Another variable which will afiect the length of time of immersion is the temperature at which the bath is maintained.

Metals other than molybdenum can also be conveniently descaled in the basic molten salt herein described. Special additives may be or may not be included to enhance the ability of the basic salt to descale these other metals. Among the other metals which can be descaled are titanium, zirconium, nickel steels, cobalt base alloys such as Vitallium and Stellite alloys, Hastelloys and other chemically resistant metals and alloys of the aforementioned metals. When working with the above metals the basic salt described herein may or may not be used electrolytically depending on individual requirements.

By virtue of the composition and temperature of the molten salt, it can be used non-electrolytically for sand and investment removal from metallic castings, removal of porcelain and glass, paints, rubbers, plastics, grease, oils and all organic soils and coatings. The inorganic materials are dissolved in the main bath and the organic materials are pyrolytically and chemically decomposed.

' By proper disposition of electrodes in the bath, the molten salt can be used electrolytically to descale the e s-so various metals mentioned herein and their alloys when the workpiece is made cathode. By making the workiece the anode in the circuit, surface carbon,--graphite and silicon can be removed, and an oxide is produced on the surface of the workpiece. If the workpiece is to be made oxide-free, the current is reversed, the oxide is reduced, and the workpiece is removed in the metallic state, usually without the need of any subsequent treatment or acid pickling.

The following are examples of compositions of this invention. They are not to be taken as all inclusive, but merely as illustrative.

Percent by weight Bath A:

Sodium hydroxide 82 to 84 Sodium carbonate 0.5 to 1.5 Sodium chloride 0.5 to 1.5 Disodium hydrogen phosphate 5 Potassium chloride 10 Bath B:

Sodium hydroxide 82 to 84 Sodium carbonate 0.5 to 1.5 Sodium chloride 0.5 to 1.5v Trisodium phosphate 5 Potassium chloride 10 Bath C:

Sodium hydroxide 82 to 84 Sodium carbonate 0.5 to 1.5 Sodium chloride 0.5 to 1.5 Monosodium hydrogen phosphate 5 Potassium hydroxide 10 In the following examples metal samples were descaled by immersing them into salt baths having a composition as indicated below. These examples are given merely to illustrate the invention.

Example 1 A sample of pure molybdenum tWo by four inches in size, and about 0.020 inch thick, having a thick scale was suspended from an 18-8 stainless steel wire by means of a small hole punched at the top. The sample was then immersed into a molten salt bath having a composition corresponding to Bath A and was maintained at a temperature of about 950 F. The sample was removed after being immersed in said bath for thirty seconds. The resulting converted scale was then quenched in a water bath, followed by hosing with an air water spray and then dried. The surface of the sample was examined and found to be bright and clean, fully descaled and not etched.

Example 2 An alloy sample containing 99.5 percent molybdenum and 0.5 percent titanium two by four inches in size and about one-sixteenth of an inch thick, having a thick dark scale was suspended in a manner after Example 1. The sample was then immersed into a molten salt bath having a composition corresponding to Bath A and was maintained at about 950 F. The sample was removed after having been immersed for a period of one minute. The resulting converted scale was then quenched in a water bath, hosed and dried. The surface of the sample was examined and found to be bright and clean, fully descaled and not etched.

Example 3 was maintained at a temperature of about 950 F. The

sample was removed after being immersed in said bath for seven minutes. It was then quenched in a'water bath, followed by hosing with an air water spray and then 3 dried. The surface of the sample was examined and found to be bright and clean, fully descaled and not etched.

Example 4 A sample of pure molybdenum twelve by twenty-six inches in size, having a dense black oxide was positioned on a stainless steel rack and suspended from an overhead crane. The sample was then immersed into a molten salt A sample of pure molybdenum three by four inches in size having a thick scale was suspended from a type 316 stainless steel wire by means of a small hole punched at the top. The sample was then immersed into a molten salt bath having a composition corresponding to Bath B and was maintained at a temperature of about 800 F. The sample was removed after being immersed in said bath for two minutes. It was then quenched in a water bath, followed by hosing with an air water spray and then dried. The surface of the sample was examined and found to be bright and clean, fully descaled and not etched.

Example 6 An alloy sample containing 99.5 percent molybdenum and 0.5 percent titanium two by three inches in size having a thick scale was suspended from a type 316 stainless steel wire' by means of a small hole punched at the top.

The sample was then immersed into a molten salt bath having a composition corresponding to Bath C and was maintained at a temperature of about 800 F. The sample was removed after being immersed in said bath for two minutes. It was then quenched in a water bath, followed by hosing with an air water spray and then dried. The surface of the sample was examined and found to be bright and clean, fully descaled and not etched.

Example 7 An alloy sample containing 94- percent titanium, 2 percent molybdenum, 2 percent chromium and 2 percent iron, two by four inches in size, having a thick black scale was suspended from an 18-8 stainless steel wire by means of a small hole punched at the top. The sample was then immersed into a molten salt bath having a composition corresponding to Bath A and was maintained at a temperature of about 850 F. The sample was removed aftcr having been immersed in said bath for a period of one minute. It was then quenched in a water bath and brightened in a conventional 10 percent nitric-12 percent hydrofluoric acid bath for thirty seconds at 100 F. The surface of the dried sample was examined and found to be bright and clean, fully descaled and not etched.

Example 8 An alloy sample containing 98.5 percent zirconium and 1.5 percent tin (Zircalloy2) having a lustrous black scale was suspended from a copper clamp and made cathode in the D.C. circuit and the pot Was made anode. The sample was then immersed into molten salt bath having a composition corresponding to Bath A and was maintained at a temperature between 800 F. to about 950 F. The current density used was approximately 240 amperes per square foot and the time of immersion was between one to one and one half hours. After the sample was removed from said bath it was quenched, dried and buffed.

The surface of the sample was examined and found to be bright and clean, fully descaled and not etched.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment of the invention is therefore to be considered as in all respects, illustrative and not restrictive, the scope of the invention being indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Having described our invention, we claim:

1. The process for removing the contaminated high oxygen-containing surface layer and annealed scale of molybdenum and its alloys, which comprises immersing the metal to be treated in a molten substantially nonoxidizing and non-reducing composition maintained at a temperature of between about 650 F. to 1250 F., comprising 50 percent to percent by weight of alkali metal hydroxide, 2 percent to 25 percent by weight selected from the group consisting of alkali metal phosphate, alkaline earth metal phosphate, and mixtures thereof, 0.5 percent to 30 percent by weight of alkali metal carbonate, 0.5 percent to 25 percent by weight selected from the group consisting of alkali metal halide, alkaline earth metal halide, and mixtures thereof, wherein the alkali metal in the above alkali metal hydroxide, alkali metal phosphate, alkali metal carbonate and alkali metal halide is selected from the group consisting of sodium, potassium, lithium, rubidium and mixtures thereof, and wherein the alkaline earth metal in the above alkaline earth metal phosphate and alkaline earth metal halide is selected from the group consisting of calcium, barium, magnesium and mixtures thereof, and continuing the immersion for at least thirty seconds, and then subjecting the sample to a water quench.

2. The process of claim 1 in which the hydroxide is sodium hydroxide, the phosphate is sodium phosphate and the carbonate is sodium carbonate.

3. The process of claim 2 in which the temperature is between about 800 F. to 1000 F.

4. The process of claim 2 in which the alkali metal halide is a mixture of sodium chloride and potassium chloride.

5. A substantially non-oxidizing and non-reducing fused metal cleaning salt bath comprising 50 percent to 90 percent by weight of alkali metal hydroxide, 2 percent to 25 percent by weight selected from the group consisting of alkali metal phosphate, alkaline earth metal phosphate, and mixtures thereof, 0.5 percent to 30 percent by weight of alkali metal carbonate, 0.5 percent to 25 percent by weight selected from the group consisting of alkali metal halide, alkaline earth metal halide, and mixtures thereof wherein the alkali metal in the above alkali metal hydroxide, alkali metal phosphate, alkali metal carbonate and alkali metal halide is selected from the group consisting of sodium, potassium, lithium, rubidium and mixtures thereof, and wherein the alkaline earth metal in the above alkaline earth metal phosphate and alkaline earth metal halide is selected from the group consisting of calcium, barium, magnesium and mixtures thereof.

6. The composition of claim 5 in which the halide is a mixture of sodium chloride and potassium chloride.

References Cited in the file of this patent UNITED STATES PATENTS 2,237,434 Holden Apr. 8, 1941 2,271,375 McKay June 27, 1942 2,380,284 Young July 10, 1945 2,395,694 Spence Feb. 26, 1946 FORElGN PATENTS 232,257 Great Britain Dec. 10, 1925 

1. THE PROCESS FOR REMOVING THE CONTAMINATED HIGH OXYGEN-CONTAINING SURFACE LAYER AND ANNEALED SCALE OF MOLYBDENUM AND ITS ALLOYS, WHICH COMPRISES IMMERSING THE METAL TO BE TREATED IN A MOLTEN SUBSTANTIALLY NONOXIDIZING AND NON-REDUCING COMPOSITION MAINTAINED AT A TEMPERATURE OF BETWEEN ABOUT 650*F. TO 1250*F., COMPRISING 50 PERCENT TO 90 PERCENT BY WEIGHT OF ALKALI METAL HYDROXIDE, 2 PERCENT TO 25 PERCENT BY WEIGHT SELECTED FROM THE GRAOUP CONSISTING OF ALKALI METAL PHOSPHATE, ALKALINE EARTH METAL PHOSPHATE, AND MIXTURES THEREOF, 0.5 PERCENT TO 30 PERCENT BY WEIGHT OF ALKALI METAL CARBONATE, 0.5 PERCENT TO 25 PERCENT BY WEIGHT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL HALIDE, ALKALINE EARTH METAL HALIDE, AND MIXTURES THEREOF, WHEREIN THE ALKALI METAL IN THE ABOVE ALKALI METAL HYDROXIDE, ALKALI METAL PHOSPHATE, ALKALI METAL CARBONATE AND ALKALINE EARTH IS SELECTED FROM THE GROUP CONSISTING OF SODIUM, POTASSIUM, LITHIUM, RUBIDIUM AND MIXTURES THEREOF, AND WHERE IN THE ALKALINE EARTH METAL IN THE ABOVE ALKALINE EARTH METAL PHOSPHATE AND ALKALINE EARTH METAL HALIDE IS SELECTED FROM THE GROUP CONSISTING OF CALCIUM, BARIUM, MAGNESIUM AND MIXTURES THEREOF, AND CONTINUING THE IMMERSION FOR AT LEAST THIRY SECONDS AND THEN SUBJECTING THE SAMPLE TO A WATER QUENCH. 